Field of the Invention
The present invention concerns a method for magnetic resonance (MR) imaging, in particular for medical applications, as well as an MR tomography apparatus for implementing such a method.
Description of the Prior Art
It is known for an MR tomography apparatus to have an array composed of a number n of single coils Ei to receive reception signals Ii. “MR” stands for “nuclear magnetic resonance”, the discovery of which is ascribed to Bloch and Purcell in 1946. An “MR tomography” apparatus executes methods for slice imaging magnetic resonance tomography (“magnetic resonance imaging”, MRI).
In general, reception coils optimized for the anatomy of an examination subject who is to be examined are used to receive MR signals generated in an MR tomography apparatus. The optimization typically takes place with regard to the resulting signal-to-noise ratio (SNR) and the capability for parallel imaging. In addition to volume coils, surface coils are used in modern MR tomography systems.
Surface coils are almost always used solely as reception coils in MR tomography, and are particularly suited to depict surface-proximate structures due to their flexible structure and small coil diameters. A volume coil integrated into the MR tomography apparatus typically acts as a transmitter. With a suitable design of the reception coils, a high signal-to-noise ratio SNR can be achieved near the coil. A disadvantage in the use of a surface coil is its small penetration depth or measurement depth, a reduced measurement field (field of view), and an inhomogeneous exposure of the image since the sensitivity of the coil decreases with increasing distance from the coil.
Modern MR tomography systems allow the subsequent correction of the intensity decline in the image using knowledge of the sensitivity profiles of the coils that are used.
A known variant depicts the combination of multiple surface coils into a coil array. A SNR gain over a larger region can be realized via the combination of multiple surface coils. Furthermore, coil arrays are a requirement for parallel imaging.
The design of such coil arrays utilizes the better SNR of smaller coils, such that a markedly better spatial resolution and/or time resolution is possible with a coil array comprising smaller single coils, as well as a larger field of view (FOV) via the simultaneous use of a multitude of these single coils. The single coils receive the MR signal of a single excitation simultaneously and independently of one another, meaning that the MR signal is acquired via independent reception channels (one coil=one channel).
A problem in this approach is that the spatial sensitivity profiles of surface reception coil arrays lead to an unintentional, spatially variable signal-to-noise ratio in the reconstructed MR image. The image intensity in proximity to the reception coil is typically increased; the image intensity then decreases with distance from the coil toward the inside of the body. The effect is more strongly pronounced the smaller the individual single coils, i.e. the larger the number of single coils in the coil array. The effect also tends to increase with increasing field strength since the reception profiles are spatially more localized.
The computational combination of the MR reception signals from the individual reception channels during the image reconstruction poses an additional problem. A phase-coherent complex addition of the signals of the different reception coils in principle offers the best signal-to-noise ratio, but if the signals are not complexly added, this can lead to spatially different signal reduction.
The problems cited in the preceding are typically handled independently of one another.
A correction of the sensitivity amplitudes is typically implemented by means of a “prescan normalize” method. Within the scope of a reference measurement, an examination subject is scanned with low resolution using two coils: a local coil array whose sensitivity profile should be corrected, and a volume coil, which is assumed to have a spatially constant sensitivity profile. MR images are respectively reconstructed from the measurement values acquired by the coil array and by the volume coil. The relationship of the two images, i.e. the respective image values, yields a spatial correction map. This approach cannot be used with current high-field apparatuses, however, because in such apparatuses a volume coil with a homogeneous reception sensitivity is not present.
Measurement signals of reception coil elements are typically combined by calculating the square of the absolute value thereof, which prevents signal cancellations but does not deliver an optimal SNR. A version known as “adaptive combine” offers an image-based method.